Why not just buy it as Desolidification, affected by "non-conventional weapons"? The player and GM can decide in-game whether a specfic weapon falls into that category. If I was GM I'd even allow a -1/2 Limited Power Limitation to be applied since the power is definitely more limited in what it works against than a standard form of Desolid.
Desolidification (affected by non-conventional weapons), Reduced Endurance (0 END; +1/2), Persistent (+1/2); Always On (-1/2), Only vs. conventional weapon damage (-1/2). 40 points.